The invention relates generally to flue gas desulfurization and, more particularly, to a method of reducing nitrogen dioxide (NO.sub.2) emissions from a flue gas duct using a dry sodium scrubbing process without significantly reducing sulfur dioxide (SO.sub.2) and nitrogen oxides (NO.sub.x) removal efficiencies.
To control acid rain the major air pollutants that must be removed from a flue gas effluent produced when burning fossil fuel are sulfur dioxide (SO.sub.2) and nitrogen oxides (NO.sub.x). In air pollution terminology, NO.sub.x refers to two gaseous oxides of nitrogen, namely, nitrogen dioxide (NO.sub.2) and nitric oxide (NO). In the past, dry sodium scrubbing has been one of the methods employed to remove SO.sub.2 and NO.sub.x. In this type of process, a dry sodium compound, such as sodium bicarbonate (NaHCO.sub.3) or sodium sesquicarbonate (Na.sub.2 CO.sub.3 NaHCO.sub.3 2H.sub.2 O), is injected into the flue gas duct that carries flue gas from the combustion chamber of a boiler or the like to the flue gas stack for venting the effluent to atmosphere. Though the sodium injection is generally effective in removing sulfur dioxide (SO.sub.2) and nitrogen oxides (NO.sub.x), during this process a portion of the nitric oxide (NO) is converted to nitrogen dioxide (NO.sub.2), which usually results in a plume emanating from the gas stack having an undesirable brown color. The chemistry of NO.sub.x removal by, for example, sodium bicarbonate is given below. EQU 2NaHCO.sub.3 .fwdarw.Na.sub.2 CO.sub.3 +CO.sub.2 +H.sub.2 O1 EQU Na.sub.2 CO.sub.3 +SO.sub.2 +NO+O.sub.2 .fwdarw.Na.sub.2 SO.sub.4 +NO.sub.2 +CO.sub.2 2 EQU Na.sub.2 CO.sub.3 +3NO.sub.2 .fwdarw.2NaNO.sub.3 +NO+CO.sub.2 3
Because the production of NO.sub.2 in reaction 2 outpaces its removal via reaction 3, the result is a net generation of NO.sub.2 with sodium bicarbonate injection.
Various additives have been used to try to eliminate the brown plume, and some success in controlling nitrogen dioxide emissions has been achieved using ammonia (NH.sub.3) and urea. However, use of these additives can create new air and waste disposal problems, including high NH.sub.3 slip and the presence of soluble NH.sub.3 compounds in the fly ash.
Hence, the invention is directed to providing a dry sodium scrubbing process that reduces nitrogen dioxide emissions to a level below that which causes the creation of a brown plume, without significantly reducing sulfur dioxide and nitrogen oxides removal efficiencies while, at the same time, avoiding the above-mentioned drawbacks and disadvantages.